An FM receiver executing the above method is on itself known e.g. from European Patent 0 333 194. The known FM receiver complies with the FM RDS broadcasting standard as defined in Specification of the Radio Data System FM RDS for VHF-FM Sound Broadcasting by the European Broadcasting Union (EBU), EBU document Tech 3244-E, March 1984 and updated in the subsequent revisions thereof. Reference is made to this document for a correct understanding of the meaning and definition of the various terms used hereinafter in connection with the FM RDS standard.
The cited FM receiver comprises a first or stationary tuner circuit and a second or scanning tuner circuit. The stationary tuner circuit is used to tune the receiver to a broadcast transmitter station with a wanted audio program, hereinafter indicated as first transmitter frequency, and to process the audio program signals for sound reproduction. If the transmitter transmits RDS data as well, then the stationary tuner circuit will also extract the RDS data carried by the received RDS transmitter signal, in particular a list of alternative frequencies (AFs). Such list provides tuning data of transmitters carrying the same program as the one the stationary tuner circuit is actually tuned to. The scanning tuner circuit is used to monitor the reception quality of the transmitter signals at each of those AFs. For this, the scanning tuner circuit is sequentially switched to each AF in the AF list to measure the fieldstrength of the transmitter signals at the respective AFs. The fieldstrength information is stored in a memory and repeatedly updated in sequential scan cycles. On deterioration of the received stationary transmitter signal, hereinafter indicated as first transmitter frequency, the stationary tuner circuit is automatically switched over in its tuning from said first transmitter frequency to another transmitter frequency, hereinafter indicated as second transmitter frequency. In the known receiver, the AF stored in the memory having highest fieldstrength is chosen to be such second transmitter frequency. However, the reception quality measurement is based on the momentary fieldstrength of the RF FM reception signal and the AF having highest fieldstrength at the moment of tuning switch over may lose this position very quickly, due to fluctuations in the environmental reception conditions. This may cause instabilities in the tuning of the receiver. Furthermore, in this known receiver, the selection of a second transmitter frequency is limited to the AFs included in the RDS AF list, being alternative frequencies carrying the same audio program as the first transmitter frequency.
An object of the invention is to improve the tuning behaviour of the above mentioned receiver.